The invention relates to elevators and, in particular, to elevators wherein the elevator cage is mounted on one or more ropes or belts by means of a pulley arrangement.
Pulley arrangements are commonly used in the elevator industry to mount and drive an elevator cage along ropes arranged within a hoistway in a building. In such an arrangement a pulley box containing two pulleys is mounted on the cage so that as the rope is driven, whether by hydraulic ram or traction sheave, it travels down along one side of the hoistway, engages with one of the pulleys deflecting it through 90°, traverses across the car, engages with the other pulley which deflects it back into the vertical plane, and travels upwards along the opposite side of the hoistway. The pulley box can be mounted to the cage at a point above the passenger car or it can be mounted below the passenger car in which case it is called and underslung arrangement. Such an arrangement is illustrated and described in U.S. Pat. No. 6,443,266.
According to ASME Code A17.1-2000, the ratio of the diameter of the pulleys to the nominal diameter of the suspension ropes should be at least 40. Hence, the diameter of each pulley is significantly larger than its width and consequently the height of the pulley box is greater that its width.
In the prior art as exemplified in FIG. 1, an upper surface 18 of the pulley box 8 is mounted to the cage either directly to a base of the passenger car or, if the car is contained within a frame, to a lower yoke of the car frame. However, in use, the majority of the vertical forces F are transferred through the lowermost portions A of the pulleys 9. Accordingly, if the pulley box 8 tilts by an angle α, a relatively large bending moment is exerted about the mounting point of the pulley box. The magnitude of the bending moment is defined by the equation M=Fx, where F is the vertical force and x is the horizontal distance between the mounting point on the upper surface 18 of the pulley box 8 and the point of application of the vertical force. F. Furthermore, the value x can be expressed as x=L cos α where L is the distance between the mounting point on the upper surface 18 and the lowermost portions A of the pulleys 9. Accordingly, for a given vertical force F and a given tilt angle α, the bending moment M is directly proportional to the value L. However, since the dimension L is principally dependent on the diameter of the pulley 9 which in turn is determined by regulatory bodies as discussed in the preceding paragraph, there is very little scope for reducing the bending moment M.
Hence, both the pulley box 8 itself and the conventional means for mounting the pulley box 8 to the cage must be capable of withstanding substantial bending moments, otherwise as the cage and the pulley box tilt, the unrestrained bending moment and the tilt angle of the pulley box would progressively increase until eventually the pulley box is permanently deformed or torn from its mounting.
This phenomenon is further exaggerated when the suspension rope is replaced by a belt as any torsion in the belt may be transferred to the pulley box to establish a bending moment about the mounting point even without any tipping of the pulley box.